This invention relates to the repair of a nickel base superalloy with a gamma prime content in an amount sufficient to result in cracking during fusion type repair. More particularly, it relates to a method for avoiding cracking during repair of an article made of such an alloy using laser fusion or welding.
Operation of modern gas turbine engines, particularly for use in aircraft, includes exposure to very high temperatures under strenuous conditions of environment and mechanical forces. To enable such operation, high temperature nickel base superalloys have been developed with high gamma prime content, for example to provide one or more of improved properties such as mechanical, castability, resistance to the strenuous operating environment, etc. To assist in providing such properties, currently used forms of such alloys are made with relatively large amounts of strengthening elements, for example including at least one of Al, Ti, V, Ta, etc. Inclusion of such amounts of strengthening elements results in the formation of relatively large amounts of the well known and widely described gamma prime phase, for example in amounts of at approximately 30 volume percent or higher. There are many commercially known and used examples of such high gamma prime content alloys. These alloys include Rene"" 80 Ni base alloy with a gamma prime content in the range of about 30-50 vol. %, forms of which are more fully described in U.S. Pat. No. 3,615,376xe2x80x94Ross et al. (patented Oct. 26, 1971); Rene"" 142 Ni base alloy with a gamma prime content in the range of about 60-70 vol. %, forms of which are more fully described in U.S. Pat. No. 4,169,742xe2x80x94Wukusik et al. (patented Oct. 2, 1979); and Rene"" N5 Ni base alloy with a gamma prime content in the range of about 60-75 vol. %, forms of which are more fully described in U.S. Pat. No. 5,173,255xe2x80x94Ross et al. (Patented Dec. 22, 1992).
Certain current fusion type repair, such as welding, of defects in turbine engine articles made from a high gamma prime content Ni base superalloy, either resulting from manufacture or from exposure to operating conditions, can result in cracking of the article. High gamma prime containing Ni base superalloys are well known to be prone to micro and/or macro cracking during welding or during post weld heat treatment (PWHT). Strain age cracking is widely believed to be the main cause of such cracking. As a result, current welding methods to repair an article, for example a worn or damaged tip of a gas turbine engine turbine blade airfoil made of a high gamma prime content Ni base alloy, requires preheating of the article to a relatively high temperature, typically above about 1400xc2x0 F., and holding at such temperature during welding, to avoid cracking. Preheating methods prior to fusion repair, such as using a laser, have been described in such U.S. Patents as U.S. Pat. No. 4,804,815xe2x80x94Everett; and U.S. Pat. Nos. 5,900,170 and 5,914,059xe2x80x94Marcin, Jr. et al.
A manual or semiautomatic Tungsten Inert Gas (TIG) or automated laser welding method generally has been used for repair build-up on such a preheated substrate. However, known methods involving preheating prior to welding, with the temperature maintained during welding, suffer from the deficiency that precise temperature control of a relatively narrow substrate such as an airfoil tip must be maintained in order to achieve good results or yield. In addition, such a repair method is relatively slow, as well as costly, because it requires equipment and time for heating to and holding during welding at the required preheating temperature. Also, such method requires that the article surface being repaired be constantly shielded from the oxidizing environment during preheating, weld repair at the preheating temperature, and cooling from the preheating temperature after welding.
The present invention, in one form, provides a laser repair method for repairing a Ni base superalloy substrate, having a gamma prime content of at least about 30 vol. %, conducted at ambient or room temperature, without the use of or need for preheating by reducing the interaction time between the substrate and a molten repair alloy to avoid cracking of the substrate during weld repair. The method comprises providing and maintaining a substrate surface at ambient temperature in juxtaposition with a laser that operates in the power range of about 50-10000 watts per square centimeter. The beam of the laser is focused at a point away from the substrate surface, sometimes referred to as defocused, to provide at the substrate surface a laser spot in the size range of about 0.03-0.2xe2x80x3 diameter, or its equivalent. The method includes, in such combination, relative movement between the substrate surface and the laser to provide an interaction time of no greater than about 10 seconds between the laser beam and the substrate surface. In one form, the rate in the range of about 1-100 inches per minute. Concurrently rather than subsequently, a powdered repair material is deposited into the laser beam, in one form at a feed rate in the range of about 0.4-15 grams per minute, into the laser spot on the substrate surface. At this combination of laser density, relative movement rate and concurrent repair powder delivery, the laser beam melts and fuses the powder into a molten repair material at a relatively low interaction time, one form of which is in the range of about 0.01-10 seconds between the laser beam and the substrate. This combination avoids cracking of a substrate highly susceptible to cracking when repairing at ambient temperature. The need for preheating the substrate before and during welding thereby is eliminated.